This invention relates to power conversion circuits.
FIG. 1 shows a zero-current switching forward converter 10 of the kind described in Vinciarelli, "Forward Converter Switching at Zero-Current," U.S. Pat. No. 4,415,959 (incorporated by reference). In such a converter a finite and bounded amount (quantum) of energy is transferred from the input source 12 to the load 200 during each converter operating cycle. As a result, the amount of power delivered to the load is a function of converter operating frequency (i.e., the number of operating cycles per second). In a typical application, where the output voltage, Vout, is to be maintained at some predetermined value, control circuits 22 are used both to turn the main switch 26 on and off at times of zero current and to vary the converter operating frequency as the power drawn by the external load 200 varies. In theory, as the power drawn by the load is reduced toward zero the operating frequency will also tend to fall to zero. In practical embodiments of such a converter, however, both the finite value of the output inductor 24 and losses in nonideal converter circuit elements will affect converter operation at light loads. Losses in circuit elements will determine the minimum average operating frequency of the converter since, at zero external load 200, the converter will process just enough power to offset the losses in the converter itself. However, at very light external loads, or at zero external load, the converter will not generally operate at a fixed, stable, operating frequency. This is because there will be some value of load below which current reversal will occur in the output inductor 24 (e.g., the current Io will flow towards the capacitor 20 during a portion of the operating cycle). Because of the unidirectional conduction characteristic of diode 18, this reverse current will cause charging of the capacitor 20 and this "precharge" on the capacitor, which will tend to vary from cycle to cycle, will affect the amount of energy transferred forward from the input source during the next operating cycle. The result is a "discontinuous" mode of operation at light loads marked by variations in forward energy transfer during different operating cycles and attendant variations in both the time periods of the operating cycles and in the converter operating frequency.
In general, it is beneficial to reduce the operating frequency range of a zero-current switching converter by raising its minimum operating frequency. For example, increasing the minimum operating frequency allows use of smaller valued converter output filter elements. This allows reducing the physical size and losses in these elements and also provides for wider closed-loop converter operating bandwidths and improved converter response times. Similarly, an increased minimum operating frequency allows use of smaller input filtering elements, for reducing the levels of the frequency components reflected back toward the input source, with similar benefits.
One way of raising the minimum operating frequency in a zero-current switching converter is to provide means for reducing the amount of energy transferred to the load during each energy transfer cycle. One way of accomplishing this is described in Vinciarelli, "Zero Current Switching Forward Power Conversion Apparatus and Method With Controllable Energy Transfer," U.S. Pat. No. 5,235,502, incorporated by reference. The converter described therein, and shown as converter 50 in FIG. 2, incorporates a bidirectional switching element 28 which is turned on and off in synchronism with the main switch in an operating mode called the reverse boost operating mode. In this operating mode, a reverse boost controller 32 turns the bidirectional switch 28 off when the main switch 26 is turned on (thereby enabling forward energy transfer to take place from the input source to the capacitor 20 and thence to the load 200) and turns the bidirectional switch on later in the operating cycle when the capacitor 20 voltage returns to zero. At relatively high values of load power, for which current in the output inductor 24 always flows in the direction of the load, the operation of the converter of FIG. 2 is substantially identical to the operation of the converter of FIG. 1. At light loads, or at zero load, however, for which current reversal in the output inductor 24 does occur, the closed bidirectional switch will conduct the reverse current around the capacitor, thereby preventing uncontrolled charging. When the main switch 26 is closed, and the bidirectional switch is opened, energy is transferred to the capacitor from two directions: "forward" energy transfer from the input source 12 and "reverse" energy transfer from the reverse current flowing in the output inductor 24. The greater the initial value of reverse current (i.e., the value of reverse current flowing in the output inductor 24 at the instant that the main switch is closed) the lower will be the amount of energy which will be transferred forward. Thus, reverse current is exploited as a means of reducing the amount of forward energy transfer. This results in a higher operating frequency at light loads (compared to the converter of FIG. 1) since, for a given value of load power, the reduction in energy-per-cycle will translate into a higher requisite operating frequency. As load is reduced toward zero, frequency will also decline, but this will result in an increase in the initial value of reverse current and a further reduction in forward energy transfer. Since the bidirectional switch and the main switch are synchronized within the operating cycle, the effects of reverse current flow are controlled and predictable on a cycle-by-cycle basis and the "discontinuous" mode is eliminated.
When the converter of FIG. 2 operates in the reverse boost mode, the peak-to-peak value of current flowing in the output inductor may become relatively large as the power drawn by the load is reduced toward zero. In practice this large current, which translates into relatively large RMS currents flowing in the output inductor, bidirectional switch and output filter capacitors (e.g., capacitor 201 in FIG. 2), can result in increased ripple voltage (due both to integrating effect of the output capacitance and to the effect of current flow in the equivalent series impedances of the nonideal output filter capacitors); will produce losses in the finite resistances in the nonideal circuit components, including the bidirectional switch; and may, in general, require use of a relatively large die size for the semiconductor switch selected to embody the bidirectional switch (to minimize power loss in the nonideal switch).
Another scheme for reverse boost is shown in Vinciarelli, "Zero-current Switching Forward Power Converter Operating in Damped Reverse Boost Mode," U.S. Pat. application 07/862,490 (incorporated by reference). In the topology described there, and shown as converter 150 in FIG. 3, a dissipative element 210 is added in series with the bidirectional switch 252 (the diode 250 is also retained to carry the current, Io, when it is flowing in the direction toward the load). The value of the dissipative element is chosen so that the dissipative element 210, in combination with the output inductor 24 and the capacitor 20, forms an approximately critically damped circuit when the bidirectional switch 252 is closed. A reverse boost controller 32 synchronizes the opening and closing of the bidirectional switch with the opening and closing of the main switch using the same protocol described above for the converter of FIG. 2 operating in reverse boost mode. In the converter of FIG. 3, however, the dissipative element acts to limit the peak (and hence the initial) value of reverse current and, in doing so, also acts to dissipate some power. The combined effect is a reduction in forward energy transfer at light or zero loads and an increase in minimum operating frequency owing to both the reduction in forward energy transfer and the losses in the resistive element. When operated in this "damped reverse boost" mode of operation, the converter of FIG. 3 avoids the higher initial values of reverse current associated with the reverse boost operating mode in the converter of FIG. 2 and eliminates the discontinuous operating mode, but does so at the expense of increased dissipation, and wasted power, in the dissipative element 210.
Both the reverse boost and the damped reverse boost operating modes exploit current reversal in the output inductor at light loads as a means of supplying a portion of the total energy delivered to the capacitor 20. This "reverse" energy transfer acts in natural opposition to "forward" transfer of energy to the capacitor from the input source. However, in each of the referenced operating modes the "reverse" energy transfer process is initiated concurrently with initiation of forward energy transfer (i.e., the bidirectional switches 28, 252 are opened by the reverse boost controller at essentially the same time that the main switches 26 are closed). With this switching protocol, reverse current flow can decrease forward energy transfer but it cannot reduce it to zero, as this would require an infinite initial value of reverse current. It is interesting to note, therefore, that if all of the components in the converter of FIG. 2 were ideal (i.e., lossless), use of the reverse boost operating mode would not prevent the converter operating frequency from approaching zero as the power drawn by the load approached zero. For practical embodiments of the converters of FIGS. 2 and 3, however, operating in the reverse boost and damped reverse boost operating modes, respectively, operation at zero external load will occur at a minimum operating frequency at which the finite amount of power transferred forward from the input source is just equal to the total of the dissipative losses in the non-ideal components in the converter (and, in the converter of FIG. 3, the losses in the dissipative element 210). The minimum operating frequency for each of the converters of FIGS. 2 and 3 is higher than the average minimum operating frequency for the converter of FIG. 1 because of the reduction in forward energy transfer at low values of load brought about by the aforementioned reverse boost operating modes. However, the minimum operating frequency is primarily dependent upon losses in nonideal or dissipative components within the converter.